The downloading of network content is often interrupted for a number of reasons, such as the user clicking on another link during the download. With progressive rendering, where a browser receives portions of the content as it is downloaded to provide partial images thereof, the user may even interrupt the download by clicking on a link that appears on a page being displayed during the download. Other reasons for the interruption of downloads include busy servers, gateways and/or service providers dropping connections, and transmission problems or the like that cause a download to be aborted.
If a user later requests download of content that was previously interrupted, the request starts over at the beginning. In general, this is because few network applications are capable of dealing with anything other than a complete response to a request for content. However, while this system works, restarting requests from the beginning has the drawback of re-transmitting data that was already sent. This increases client latency, particularly when the client is communicating over a low-bandwidth connection, increases the load on the server and takes up available network resources (e.g., available bandwidth).